Insulin resistance is a major contributor to the epidemic of metabolic diseases including dyslipidemia, hypertension and type 2 diabetes. According to the CDC, approximately 10% of U.S. adults have diabetes now and 33% are expected to have diabetes by 2050. Therefore, there is a serious demand for the development of new therapeutics to increase insulin sensitivity. Fibroblast growth factor 21 (FGF21) is an endocrine hormone that is a potent insulin sensitizer. Unlike other insulin sensitizers that indirectly improve insulin sensitivity by reducing ectopic lipid accumulation, FGF21 has the unique ability to directly enhance insulin sensitivity. We recently demonstrated that circulating FGF21 levels are produced from the liver during overfeeding and function to enhance insulin-stimulated glucose disposal in brown adipose tissue (BAT). The aims of this grant are to 1) determine the contribution of FGF21 action on BAT to the insulin-sensitizing effects of FGF21, and 2) determine the mechanism by which FGF21 enhances insulin sensitivity in brown adipocytes. To accomplish these aims, we propose several experimental approaches. In specific aim 1, we will utilize novel animal models to inactivate, reactivate or constitutively activate FGF21 signaling to specific adipose depots in vivo to access tissue-specific effects on insulin sensitivity. In specific aim 2, we will examine the mechanism whereby FGF21 enhances insulin-stimulated glucose uptake in brown adipocytes. In addition, we will evaluate the contribution of insulin-stimulated glucose uptake in BAT to the glucose lowering effects of FGF21 in vivo. These studies are significant because they will provide new information on the mechanism of FGF21 action, and also provide fundamental insight into the mechanisms regulating whole-body glucose homeostasis.